There are many needs that can be addressed by a very thin membrane material of high uniformity and high tensile strength. One example is the field of transverse electron microscopy (TEM), wherein a TEM grid supporting a taut, extremely thin, and uniform membrane is required to provide specimen support to enable high resolution, high contrast uniformity TEM imaging. The membrane must be free-standing or unsupported in the open regions between grid supports and should also provide adequate electrical conductivity to avoid surface charging and/or damage under electron irradiation.
TEM grids are typically 3 mm copper or silicon discs with arrays of photo-etched holes in their surface so that the TEM electron beam may pass through the hole and be received by the image detector. The objects being imaged are typically supported by only the membrane in the grid holes, so they may be fully imaged, or larger objects may be supported by both the grid and the membrane. Sometimes thin polymer membranes have been used to cover the grid and support the objects to be imaged. In some cases a graphene or lacy carbon film may be used over the grid (either supported or not supported by a polymer film). A carbon film may be supported by a polymer film that is subsequently removed (by dissolution or otherwise). Further, enhanced materials for TEM support membranes have been developed with chemical resistance to particular solvents etc. used during the sample preparation so as not to contaminate the specimen itself via chemical mixing. Carbon films deposited on top of a sacrificial membrane can be very thin (5-50 nanometers) and typically provide adequate chemical inertness together with reasonable uniformity for most TEM imaging. As TEM imaging continues to evolve to higher resolutions and higher magnifications, thinner carbon films (less than about 3 nanometers) and graphene layers that both require a lacey carbon support layer to provide adequate strength for specimen support have been developed in hopes of meeting increasingly stringent demands. These support mechanisms are typically fabricated using layer transfer techniques and have poor yield of usable grid space due to non-uniformity caused by membrane wrinkling, non-uniformity of layer thickness (which causes imaging contrast difficulties), and lack of adhesion to the TEM grid itself which can result in a slack support membrane of low strength and low yield coverage of the grid openings. The use of a lacey carbon support layer further complicates matters as it absorbs the TEM beam as well and so the ultimate usable area of these high-end grids TEM imaging is not always suitable for some modern needs.
To create a useful free-standing support membrane for modern high resolution TEM needs, the following criteria should be met. First, the support membrane must have the mechanical strength to support the specimen and provide a taut surface without wrinkles, holes, or mechanical defects, and should provide a high usable yield of the grid openings. Secondly, the membrane should be adequately inert to the chemicals used during sample preparation to avoid contamination of the specimen itself. Third, the uniformity of the membrane thickness must be tightly controlled so that electron absorption and scatter of the TEM beam is minimized and the background contrast of the beam is homogenous to provide as much usable contrast for specimen analysis as possible. Lastly, the membrane material must be able to withstand the rigors of the TEM process itself. The membrane should be at least somewhat electrically conductive to avoid electrical charging problems when subjected to the TEM beam and must withstand electron bombardment conditions. A very thin carbon film with uniform thickness would suit the purpose, but other membrane materials are also possible.
TEM grids with sacrificial support membranes of various type and compound are known in the prior art. A commonly employed polymer material for a support membrane is known by the trade name FORMVAR® (or VINYLEC®). It may be employed as a support membrane or as a sacrificial support. Other membrane materials also exist and are used for similar purposes. Polymer materials do not always withstand the TEM beam and can burn and deform during TEM beam exposure, contaminating the specimen material, so they are often only employed as sacrificial supports that are removed by dissolution prior to TEM imaging.
Another example of a need that can be satisfied by a very thin, free-standing membrane material of high uniformity and high tensile strength is in the field of pellicles for photomasks and reticles used in projection imaging, etc. In this field, a free-standing membrane supported at a distance from an optical element (photomask, etc.) protects an optical surface from particulate contamination and retains contaminating particles at a position where they are outside of the focal plane for projection imaging. Such membranes require high transparency at the projection wavelengths, high uniformity, and adequate strength to withstand required handling. Historically polymer films such as, for example, polyester film, PTFE, FEP, PFA, etc. have been employed.
Other examples of need will be known to practitioners upon understanding the characteristics and benefits of the invention disclosed below.
It is therefore an object of this invention to provide a very thin, uniform thickness, high-tensile-strength free-standing film for use as a TEM support membrane, a pellicle, or in another requirement demanding such a film.
It is another object of this invention to provide methods for forming a thin, uniform, high-tensile-strength free-standing film by conversion of a surface layer of a polymer using Neutral Beam irradiation, with subsequent removal of any remaining polymer.
A further object of this invention is to provide methods for attaching a thin, uniform, high-tensile-strength free-standing film to a support such as a TEM grid or a pellicle support.